feat: WS 每 IP 并发连接上限（默认10，(租户,IP) 键，finally 释放）

Co-Authored-By: Claude Code <noreply@anthropic.com>

app/engine/routes/chat.py

@@ -5,6 +5,8 @@ from fastapi import APIRouter, WebSocket, WebSocketDisconnect
 
 from app.engine.models import ConversationSource, ConversationStatus
 from app.engine.routes.runtime import get_runtime
+from app.engine.services.conn_limiter import DEFAULT_WS_CONN_LIMIT
+from app.engine.services.conn_limiter import limiter as conn_limiter
 from app.engine.services.conversation import ConversationManager
 
 logger = logging.getLogger(__name__)
@@ -13,21 +15,32 @@ router = APIRouter()
 
 @router.websocket("/ws/chat")
 async def ws_chat(websocket: WebSocket):
-    await websocket.accept()
+    # Task 8: 每 (租户, IP) 并发连接上限——必须在 accept 前拒绝(close 代
+    # accept,客户端握手即断)。IP 提取与下方 client_info 同一来源(XFF 优先,
+    # scope client 兜底),握手期 headers/scope 已就绪、不依赖首条 init 消息;
+    # 客户端自报的 client_info.ip 仍按原逻辑只进会话记录,不参与限流。
+    runtime = get_runtime(websocket)
+    client_ip = websocket.client.host if websocket.client else ""
+    forwarded = websocket.headers.get("x-forwarded-for", "").split(",")[0].strip()
+    if forwarded:
+        client_ip = forwarded
+
+    limit = runtime.config.data["engine"].get(
+        "ws_conn_limit", DEFAULT_WS_CONN_LIMIT)
+    if not conn_limiter.try_acquire(runtime.config.slug, client_ip, limit=limit):
+        await websocket.close(code=1013, reason="too many connections")
+        return
+
     session_id = None
 
     try:
+        await websocket.accept()
         # Task 5b: 租户运行时经 state/默认租户兜底解析(过渡期无中间件);
         # manager 由装配线从 runtime 造(db 会话 + runtime 引用)
-        runtime = get_runtime(websocket)
         async with runtime.session_factory() as db:
             manager = ConversationManager(db, runtime=runtime)
 
-            # Collect client info
-            client_ip = websocket.client.host if websocket.client else ""
-            forwarded = websocket.headers.get("x-forwarded-for", "").split(",")[0].strip()
-            if forwarded:
-                client_ip = forwarded
+            # Collect client info(client_ip/forwarded 已在握手期提取,复用)
             user_agent = websocket.headers.get("user-agent", "")
             client_info = {"ip": client_ip, "user_agent": user_agent}
 
@@ -107,3 +120,7 @@ async def ws_chat(websocket: WebSocket):
         logger.info("Client disconnected: %s", session_id)
     except Exception:
         logger.exception("WebSocket error for session %s", session_id)
+    finally:
+        # Task 8: 正常关闭与异常断开都归还名额——acquire 成功后所有路径
+        # 必经此处,不泄漏(accept 失败/循环中抛错同样覆盖)
+        conn_limiter.release(runtime.config.slug, client_ip)